1. Field of the Invention
The present invention relates to a high-frequency signal line, and more particularly to a high-frequency signal line preferably for use in high-frequency signal transmission.
2. Description of the Related Art
As a conventional high-frequency signal line, for example, a signal line disclosed in WO 2011/007660 is known. The signal line includes a laminate body, a signal line and two ground conductors. The laminate body is formed by laminating insulating sheets. The signal line is embedded in the laminate body. The two ground conductors are arranged in the laminate body so as to sandwich the signal line from both sides in the direction of lamination. Accordingly, the signal line and the two ground conductors form a triplate stripline structure.
Each of the ground conductors has openings in portions over the signal line when viewed from the direction of lamination. In the portions where the openings are located, less capacitance is created between the signal line and each of the ground conductors. Thereby, it is possible to shorten the distance in the lamination of direction between the signal line and each of the ground conductors without causing too large a reduction in the characteristic impedance of the signal line. Accordingly, it is possible to make the high-frequency signal line thinner. Such a high-frequency signal line is used, for example, to connect two circuit boards.
The high-frequency signal line disclosed by WO 2011/007660, however, has a problem that the interval between the signal line and each of the ground conductors varies depending on the conditions at the time of pressure-bonding the laminate body. Specifically, in the pressure-bonding process of the high-frequency signal line, heat and pressure are applied to the laminate of insulating sheets. By this process, the insulating sheets, which are formed of liquid polymer, polyimide or the like, are softened, and adjacent insulating sheets in the direction of lamination are joined together.
However, if the heating condition and the pressing condition of the pressure-bonding process vary, the softening degree of the insulating sheets will vary. Consequently, among laminate bodies produced by the pressure-bonding process, the thicknesses of the insulating sheets vary, and the interval between the signal line and each of the ground conductors vary.